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Building Retaining Walls

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Retaining walls are a great addition to any landscape environment particularly those sites with a slope; the retaining wall effectively levels even the hardest of inclines or declines, allowing you to transform an empty slope into an enclosed area for outdoor leisure or a decorative garden.

A retaining wall is a structure that holds or retains earth behind it.

The most important consideration in proper design and installation of retaining walls is to counteract the tendency of the retained material to move downslope due to gravity. Earth pressures will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes enormous hydrostatic pressure on the wall.

There are many types of materials that can be used to create retaining walls like:

  • Concrete blocks
  • Poured concrete
  • Treated timbers
  • Rock
  • Boulders
  • Railway sleepers
  • Bricks
  • Gabion baskets

Some are easy to use, others have a shorter life span, but all retain earth.

We need to know what type of soil we are retaining before we will know what type and height of retaining wall we can build.

For example, is the soil: sand, clay, or something in between?

What is behind the wall: a slope, a driveway, a garden or something much heavier like a pool?

Will the ground above or below the wall be level or sloped?

What will be the height of the wall?

All of these factors will determine whether we will need to build a gravity retaining wall or a reinforced retaining wall.

Gravity retaining walls
Gravity retaining walls are those that depend on their own weight and setback/batter (the distance a course will offset back into the slope from the course beneath it) and are typically shorter in height. A good example is a gabion wall, which is effectively, caged rock.

Reinforced retaining walls
Reinforced retaining walls use some type of reinforcement to give more strength to the retaining wall. One common method is to place geotextiles (a manufactured high strength reinforcement grid material that comes in rolls of various sizes and strengths) between courses of blocks in the wall and rolled back into the slope during installation to create a stronger and more stable soil mass.

Sometimes this method is not a usable solution on a project due to space or other site conditions, so alternative reinforcement materials need to be used – such as soil nails or earth anchors; usually required in big commercial projects with very steep slopes and limited space. These types of reinforcement require the services of a local geotechnical engineer for the wall’s design.

Timber is a great material for retaining walls being cost effective and fast to erect. Timber is a good choice for low level garden walls and edging particularly where a natural look is important.

It is important to take care in selecting the timber product and also in the installation so as to achieve a long lasting result. Hardwood sleepers should be selected to ensure that the species is durable or has been preservative treated so that the sapwood is protected, railway sleepers are still available and give a rustic look. Preservative treated pine is the most popular choice because of its availability and relatively low cost, however care should be taken to select sleepers that meet the Australian Standard and are clearly labelled.

The preservative class for most available sleepers is H4 being a light duty in-ground classification that is suitable for low level retaining walls up to one metre in height. Where the retaining wall is over one metre or is in a more critical situation then H5 classification as well as structurally graded sleepers or rounds must be used in conjunction with engineering approval. Lastly use the best quality hot dipped galvanised, stainless or treated pine fasteners and fixings and don’t forget to use a remedial preservative on cut ends.

Stones, rocks and boulders can create beautiful retaining walls. Be mindful though that their installation is labour intensive.

Dry stack walls can typically be made from three types of stones: round field stones, relatively flat stacking stones and uniformly cut dressed stones. Each type will give our wall a different look and feel. Flatter stones tend to be easier to work with than rounder ones, as they are more suited for stacking. Footings should be as wide as the wall is tall; so we may need plenty of room.

As we build the wall, we toss all unwanted stone scraps behind the wall. When the wall is complete, backfill behind it with gravel to just below the level of the cap stones or top course. For a more secure top, some landscapers set the cap stones into a bed of premixed mortar.

Importantly, these walls are self-draining.

Mortared retaining walls
Mortared retaining walls are another option. These walls can be installed with a variety of veneers, bricks, flagstones, etc. which will add colour, style and texture to any landscape design.

Good mortar is of vital importance in all brick or block walls as it bonds the units together, helps to carry the weight placed on the wall and seals the joints to provide a weatherproof wall.

Lime is added to make the mortar creamier or more workable and durable. It also helps to minimise cracking as the mix dries out.

Aggregate sand is the fine aggregate component which is the basis of the mortar and only recognised brickie’s sand should be used.

Water for use in making mortars should be clean, fresh and free from impurities. As a general rule, if the water is suitable for drinking it will be okay for making mortar. Pigments added to colour the mortar should not exceed 10 per cent of the weight of cement in the mix and should be thoroughly mixed with the other materials prior to the addition of water.

Segmental retaining walls
Segmental retaining walls offer concrete masonry blocks that are modular and inter-locking for ease of use. Simply stack the mortarless blocks together and create a maintenance-free retaining wall. These types of retaining walls use similar construction techniques as found in the mortarless construction of the Great Wall of China.

Segmental retaining walls come in a variety of sizes, shapes and colours as well as different facing textures. Concrete blocks made from recycled materials are a sustainable option. You can hunt out supplies through online resale shopping sites such as Ebay® or Gumtree®.

A slope above a retaining wall will add more pressure and weight, while slopes below the retaining wall may make the wall unstable due to sliding or erosion. Avoid slopes greater than three to one without first consulting with a local geotechnical engineer.

Any added weight behind a wall is called a surcharge. Patios, swimming pools and driveways are common residential surcharges. Get a geotechnical engineer’s opinion in these instances.

Water in all its forms is a major challenge for landscapers on a site. The enormous pressure on a wall due to water can be destructive so it’s imperative we deal with this potential problem before it destroys our work and reputation.

The design and performance of most retaining walls are based on keeping the area behind the wall relatively dry. During wall layout it is important to evaluate the entire site to determine if water will drain into the area where the wall will be constructed. Temporary grading of the site may be needed to ensure water will not drain towards the construction site. This may entail a discussion with the client and landscape designer to achieve the best outcome.

Gabion Basket Retaining Walls
Gabions are an attractive option for retaining works where longevity and visual presen-tation are important considerations. Gabions can comprise any form of metal mesh which is filled with rocks. Usually, welded galvanised steel reinforcing mesh wire offers a lengthy life span and depending on the gauge of the steel and the size of the aperture, a better shape can be achieved.

Like any form of retaining wall, drainage is required behind the wall and this must be supported by geotextile fabric to avoid dirt and dirty water passing through the gabion particularly when maintaining the appearance at the face of the wall ie. where the retaining wall abuts say, paving.

Various stone products or even bricks can be used to fill gabions. Rubble can be poured into the gabions where retention is the only consideration, while often stone is stacked at the face to present a neater and cleaner look where appearance is a consideration.

Various stone products or even bricks can be used to fill gabions. Rubble can be poured into the gabions where retention is the only consideration, while often stone is stacked at the face to present a neater and cleaner look where appearance is a consideration.

Drainage
Prior to constructing the wall, we review our drainage plans to identify all potential sources of concentrated water such as driveways, slopes above walls, grading of site, water lines, mains or fire hydrants, roof downpipes and irrigation systems. Perhaps there may be a need to employ the services of a hydraulic or civil engineer to design the retaining wall when water plays a critical part.

NB: By their very nature, drystone retaining walls have excellent drainage. The Dry Stone Walls Association of Australia have a great website that provides more details on this. Visit www.dswaa.org.au to find out more.

To ensure a quality project, the soils used must not become saturated during construction and the final design must route water away from the back of the wall. To this end, we always incorporate a toe drain and weepholes in the wall as the minimum requirements. Weepholes are small openings through the wall that allow water to seep out rather than build up behind the wall. Face the backside of the weep holes with Geotech to block dirt but let water drain through. Vary the number and spacing of weep holes depending on the soil type; for instance, sandy soil may not require weep holes. Use metal or pvc pipes in the mortar joints or similar gaps in the wall.

In the first instance, prior to quoting , always check requirements and regulations with your local council or certifier

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